1. Field of the Invention
The present invention relates generally to an image forming apparatus such as a copying machine and a printer etc, and more particularly to an image forming apparatus using a charging rotary body coming into contact with an image bearing body.
2. Related Background Art
An image forming apparatus such as a copying machine or a printer etc. adopting an image forming method based on an electrophotographic system, has hitherto been constructed to form an image generally by a charging step of equally uniformly charging an image bearing body with electricity, a latent image forming step of writing an electrostatic latent image onto the image bearing body, a developing step of developing the electrostatic latent image with a toner, a transferring step of transferring the toner on the image bearing body onto a transfer material, a fixing step of fixing the toner on the transfer material, and a cleaning step of cleaning a residue such as transfer residual toner remaining on the image bearing body after the transfer ring step.
In the construction described above, a disposal-toner container contains the toner collected by the cleaning step, executed after the transferring step, of cleaning the residue on the photosensitive drum such as the transfer residual toner remaining on the image bearing body.
A charging unit for charging the image bearing body involves the use of a wide-spread contact charging system using a charging member brought into press-contact with the image bearing body.
The contact charging system has advantages such as saving the electric power and generating just a small amount of ozone.
What is predominant among the contact charging systems is a system using a charging roll in terms of a durability etc.
Further, there has been in recent years proposed a cleanerless process for actualizing an extremely efficient image forming apparatus by attaining an omission of collecting the on-the-photosensitive-drum residue such as the transfer residual toner in the cleaning step described above, a down-size of the image forming apparatus and an omission of the maintenance such as disposing of the toner and so forth.
In the cleanerless process, a developing device adheres the toner with a reversal developing to a portion where a surface potential is attenuated upon the photosensitive drum being exposed to the light, and collects the on-the-photosensitive-drum residue such as the transfer residual toner remaining on non-exposed portions.
More specifically, after the transferring step, the on-the-photosensitive-drum residue such as the transfer residual toner is, after passing through the charging step, collected by the developing device by power of static electricity due to a difference between the surface potential of the image bearing body and a developing bias (which is referred to as a back-contrast).
FIG. 8 shows an example of the apparatus using the cleanerless process.
Referring to FIG. 8, a charging roller 12 charges a photosensitive drum 1 classified as a rotary drum type electrophotographic photosensitive body serving as a charged body and an image bearing body as well.
Further, the charging system involves the use of a method of applying a DC voltage of xe2x88x921300 V to a core bar of the charging roller 12 and charging the photosensitive drum 1 in contact with the drum 1, wherein a charging potential (a dark area potential) of the photosensitive drum 1 is set to xe2x88x92700 V.
Referring again to FIG. 8, a laser beam 9 is emitted from an unillustrated exposing unit on the photosensitive drum 1 to form thereon an electrostatic latent image of image information.
Subsequently, the latent image is developed with the toner by the developing roller 2 of the developing device. Simultaneously, in the present image forming apparatus incorporating no cleaning device, the developing roller 2 collects a residue 10 on the photosensitive drum 1 such as transfer residual toner remaining on the photosensitive drum 1.
The developed image is transferred onto a transfer material 5 led in between the photosensitive drum 1 and a transfer roller 4 at a proper timing from an unillustrated sheet supply unit.
A layer thickness regulating member 3 composed of a urethane rubber or a metal plate, of which a proximal end is fitted to a developer container, comes into elastic-contact with the developing roller 2, thus regulating a layer thickness of the toner on the surface of the developing roller 2 to a predetermined uniform value. An agitating member 7 provided in a toner hopper 6 serving as a container rotates, thereby supplying a predetermined quantity of toner to the developing roller 2.
The transfer material 5 passing through the transfer unit is separated from the photosensitive drum 1, then conveyed to an unillustrated image transfer unit, and repeatedly used for forming the image.
For collecting the residue 10 on the photosensitive drum 1 such as the transfer residual toner due to the back-contrast described above, it is required that the residue 10 on the photosensitive drum 1 such as the transfer residual toner charged to a positive polarity when in the transferring step, be charged to a negative polarity.
It proved from an experiment that according to the method of applying only the DC voltage to the core bar of the charging roller 12 and thus charging the surface of the photosensitive drum 1 while coming into contact with the photosensitive drum 1, the polarity of the residue 10 such as the transfer residual toner can be reversed from positive to negative to some extent during the charging step, however, a part of the residue 10 such as the transfer residual toner, which is insufficient in terms of reversion of its polarity, is nonuniformly adhered (as a contaminant 11 adhered onto the charging roller) to the surface layer of the charging roller 12. This conduces to a problem of causing a decline of charging uniformity.
As described above, the contact charging system has the problem that the charging member is contaminated and declines in terms of its charging uniformity. This problem is serious in the cleanerless process.
It is a primary object of the present invention to provide an image forming apparatus capable of preventing a charging member coming into contact with an image bearing body from being partially contaminated and effecting nonuniform charging.
It is another object of the present invention to provide an image forming apparatus capable of dispersing a contamination onto a charging rotary member by a slide friction sheet.
It is still another object of the present invention to provide an image forming apparatus capable of reciprocating the slide friction sheet.
Further objects of the present invention will be apparent in the following explanation.